The present invention relates to a magnetic flux detecting circuit which uses a SQUID fluxmeter, and more particularly to a magnetic flux detecting circuit adapted to detecting nuclear magnetic resonance signals.
In recent years, it has been attempted to use a SQUID (superconducting quantum interference device) fluxmeter as a magnetic flux detector for a nuclear magnetic resonance (NMR) apparatus or a magnetic resonance imaging (MRI) apparatus. An example which is applied to the MRI apparatus has been disclosed in Japanese Patent Laid-Open No. 60-143752 (1985).
In the conventional art, the generated MRI signals are detected by a magnetic flux detecting coil of a quadratic differentiation type, and are sent to a SQUID via a transfer circuit made of normally conducting matter.
The SQUID used in the conventional art is called dc-SQUID and its V-.phi. characteristics establish a periodic function in relation to .phi. as shown in FIG. 8, where symbol V denotes an output voltage of the SQUID and .phi. denotes a magnetic flux input to the SQUID.
The SQUID fluxmeter of the above prior art uses a FLL (flux locked loop) in order to impart linearity to the relationship between the magnetic flux and the voltage.
Theoretically, the magnitude of magnetic flux signals of MRI and NMR varies in proportion to the square of the frequency of the signals. Further, the frequency of the signals vary in proportion to the intensity of a static magnetic field applied to a body or a sample to be tested. In order to improve the S/N ratio .of signals by increasing the magnitude of signals, therefore, it is necessary to intensify the static magnetic field and to increase the resonance frequency. With the conventional SQUID fluxmeter using FLL, however, it was not able to detect magnetic flux signals of a frequency in excess of 100 KHz due to limitation on the response characteristics of the FLL.
Magnetic flux detecting circuits based on the SQUID without using FLL have been disclosed in, for example, Applied Physics Letters, Vol. 47(6), pp. 637-639 and in U.S. Pat. No. 4,875,010 having some of the same inventors as the present application. The former example teaches an art in which a capacitor is inserted in a superconductive loop that couples a pick-up coil to the SQUID such that the loop works as a resonance circuit, and in which a Q spoiler is further inserted in the loop. The latter example teaches an art in which part of the superconductive loop is rendered to be normally conductive during the periods other than a period in which a desired signal is generated, in order to suppress undesired noise signals by the resistance.